420 AN INTRODUCTION TO ZOOLOGY 



In a previous chapter the manner in which the blastoderm 

 spreads out over the yolk has been described, and this expansion 

 has been going on at the same time as the above phenomenon. The 

 result is that the ring of peripheral periblast, with its accompanying 

 sinus terminalis, has passed a good way round the yolk by the time 

 the formation of the amnion is completed. The actual growing 

 edge is formed by the union of ecto- and entoderm, but it is closely 

 followed by the mesoderm, which splits as it progresses into 

 splanchnic and somatic layers separated by extra- embryonal 

 ccelom. This process is not completed until the twelfth day, and 

 then, as will be realised, the yolk will be enclosed in a bag composed 

 on the inside of entoderm and on the outside of splanchnic meso- 

 derm. The yolk sac, as this structure is called, is separated from the 

 outside layer of the blastoderm, composed of ectoderm and somatic 

 mesoderm, by the extra-embryonic ccelom. Long before the com- 

 pletion of this process this layer over the yolk, the splanchnic 

 mesoderm and entoderm, or splanchnopleure, is spoken of as the yolk 

 sac, and from its walls a series of unfoldings, the yolk sac septa, 

 arise. At first its connection with the embryonic gut is a wide one, 

 but it gradually becomes restricted with the formation of the 

 limiting sulci, the fore and hind guts, and the fusion of the amnio- 

 cardiac vesicles. Independently of these, however, the splanchno- 

 pleure continues to constrict, until finally it is reduced to a narrow 

 tube, the yolk stalk, putting the cavity of the sac in communication 

 with the lumen of the gut in the region of the vitelline veins. Some- 

 what later the limiting sulci, i.e. the somatopleure, composed of 

 ectoderm and somatic mesoderm, also constricts, until it leaves only 

 a narrow opening, the umbilicus, around the yolk stalk. 



The allantois is entirely different from the other membranes 

 we have discussed, inasmuch as it is of embryonal origin. We have 

 seen that the tail fold brings with it the formation of a short hind 

 gut, which later becomes lengthened, not only by direct growth, but 

 also by the processes resulting in the formation of the yolk stalk and 

 umbilicus. At an early stage a ventral depression appears in the 

 floor of the hind gut, and this grows outwards as a tubular structure 

 from the tail fold bay into the extra-embryonal ccelom. It has just 

 commenced to do this by the end of the third day. It will be 

 noticed that from the beginning it is composed of entoderm in- 

 ternally, ectoderm externally, and mesoderm between them : the 

 latter being continuous with the somatic mesoderm of the embryo. 

 After a short while the outgrowth develops a characteristic arrange- 

 ment of blood-vessels. Later, the proximal part of this outgrowth, 

 which remains narrow, is termed the allantoic stalk, while the distal 

 portion reaches the chorion and spreads out to form an enormous 



